1. Field of the Invention
The present invention relates to an electrode using a three-dimensional porous metal substrate, a cell using the electrode and a process for producing an electrode for a cell.
2. Description of the Related Art
One of the electrode structures for an alkali storage battery includes a three-dimensional porous metal substrate, such as foamed nickel filled in an active material. An electrode having such a structure has reduced strength and electrical conductivity in its substrate, as compared with a sintered electrode having a core metal plate. This type of an electrode is produced by a process consisting of filling a substrate entirely with an active material, removing the active material from a part of the substrate by applying an ultrasonic vibration or the like, welding a collector tab to the part of the substrate, and then connecting the substrate to an electrode terminal via the collector tab.
FIGS. 3A and 3B are sectional views illustrating an example of the structure comprising the foregoing electrode connected to an electrode terminal. FIG. 3A is a sectional view obtained by cutting the electrode parallel to the surface of the electrode. FIG. 3B is a sectional view obtained by cutting the electrode perpendicularly to the surface of the electrode. A collector tab 7 is welded to a metal terminal plate 4 having a rising portion that is provided inside the cell. The metal terminal plate 4 having a rising portion provided inside the cell is connected to a positive electrode terminal 1. A collector tab 7 is connected to the metal terminal plate 4. In this arrangement, an electrode 5 is connected to the metal terminal plate 4 via the collector tab 7. In FIGS. 3A and 3B, the shaded portion indicates the portion of the electrode 5 where an active material has been removed. Reference numerals 2 and 3 represent an insulating packing and a cell case, respectively.
A cell needs to satisfy requirements that it has a capacity as high as possible and is provided at a low price. The foregoing process involves the removal of an active material from a part of the substrate, and then, a collector tab is connected thereto. Accordingly, the process cannot provide effective utilization of a region from an active material that has been removed, preventing the enhancement of capacity. The additional process of attaching a collector tab adds to the production cost.
Further, this type of an electrode can be produced using a relatively easy process. This type of an electrode can provide good electricity-collecting properties even if an active material having a low electrical conductivity is used. This type of an electrode is also advantageous in that it can provide a cell having an enhanced capacity.
However, a collector leading portion for conducting current can hardly be provided on the electrode prior to the filling of an active material. Therefore, a process normally employed includes filling the entire substrate with an active material, removing the active material from the substrate at a predetermined area, and then connecting a collector member to the area to form a collector leading portion.
If a collector lead is connected by this process, the active material or a resin used for retaining the active material must be completely removed from the surface of the substrate. As a preferred process for removing an active material, ultrasonic vibration is applied to the substrate at a predetermined point.
In order to produce a cell having a capacity density as large as possible, the substrate should be filled as much as possible with an active material. In order to meet this demand, various countermeasures have been worked out. For example, the distribution of particle diameters of active material is adjusted. The amount of binder resin is reduced. A three-dimensional porous metal substrate skeleton is finely divided. The weight of the three-dimensional porous metal substrate is reduced. The three-dimensional porous metal substrate filled with an active material is reduced. The pressing pressure is increased in an electrode containing the foregoing three-dimensional porous metal substrate filled with an active material. These countermeasures have successfully made it possible to gradually increase the cell capacity.
On the other hand, however, as the packing density of active material filler in the substrate increases, there tends to be an increase in the percentage of defects occurring during the production process. The reason for this phenomenon was investigated and it was found that as the packing density increases, the active material can be retained more rigidly in the substrate. Thus, the active material can hardly be removed by physical methods, such as ultrasonic vibration. The active material cannot be completely removed from the predetermined surface of the substrate. Thus, the collector member connected to the substrate suffers from a deteriorated connection strength. As a result, a defect such as disconnection of the collector member can occur at the subsequent process of cell assembly.
In order to inhibit the occurrence of such a defect, some countermeasures have been studied. Namely, the intensity of ultrasonic vibration is increased during the removal of active material, and the energy for spot welding or ultrasonic welding for connection of collector member is increased.
However, the approach involving the increase of the intensity of ultrasonic vibration during the removal of active material causes the following problems. For example, the three-dimensional porous metal substrate can be damaged. The strength of the substrate deteriorates where the collector member is connected. As a result, cutting the substrate around the connection point causes the collector member to fall off. Thus, this approach cannot give a good solution to the problems. Further, the approach involving increasing the energy for connection of collector member has the following disadvantages. Namely, since the strength of the substrate or the like is limited, the intensity of energy to be applied cannot be raised beyond some value. Further, if the intensity of energy is raised beyond some value, energy is wasted. Thus, this approach cannot give a good solution to the foregoing problems. Moreover, an approach has been studied for removing a binder resin alone with a solvent to reduce the retaining force of an active material, and then removing the active material. However, since a high capacity electrode contains a very large amount of a binder to have an enhanced packing density of active material and the majority of the retaining force of the active material greatly depends on the pressure developed by pressing the substrate, this approach involving the treatment of the binder exerts little effect.